1. Field
This technology relates generally to oxidation protection and, more particularly, to oxidation protection for carbon composites.
2. Background Art
Carbon-carbon (C/C) composite brakes are one third the weight of typical steel brakes, and they attain strength and frictional properties at temperatures up to 1600° C. C/C brakes can endure high temperatures, but in the presence of oxygen they will begin to oxidize at 400° C. Anti-oxidant systems must be applied to the non-rubbing surfaces of the C/C composite stators and rotors to prevent oxidation. Currently, commercial phosphorus based coating materials are made of crystalline metal phosphates that are derived from heat treated phosphoric acid-based liquid precursors painted on the non-rubbing surface of C/C composites [1,3,5]. These crystalline metal phosphate coatings are very porous and tend to move to the friction surface when exposed to increased levels of relative humidity.
This anti-oxidant migration towards the rubbing surface causes a drop in frictional properties. To improve the performance and stabilize phosphorus based systems, other factors such as the crystal structure, glass transition temperature, and the re-crystallization of glass products at elevated temperatures must be taken into consideration along with the opening of pores within the composite [4]. Adjusting the amounts of glass formers, glass network modifiers, and glass intermediates will alter the coatings performance and stability by changing the chemistry of the phosphorus based glass. Glass formers used in the anti-oxidant systems include phosphorus and boron because they can resist some catalytic oxidation effects, inhibit oxidation, and exhibit selfhealing capabilities which is great for constant cyclic conditions. Glass network modifiers found in anti-oxidants are commonly potassium, sodium, calcium, and manganese elements. Typical glass intermediates are aluminum and zinc oxides [5].
Carbon fiber friction materials have great retention of strength and stiffness at elevated temperatures, but begin to oxidize when exposed to air at temperatures at or above 400° C. [6].
Current barrier coatings are typically classified by their functional temperature ranges of oxidation protection into two classifications: high temperature coatings and low to moderate temperature coatings. Aoki [1,9] studied through thickness cracks in high temperature SiC coatings. Due to a large mismatch in the thermal expansion coefficients of the SiC surface layer and the C/C composite, many cracks form in the surface layer which leads to severe oxidation-degradation [1,9]. Walker patented a multilayer protection system for C/C aircraft brakes which was comprised of a SiC coating on top of a phosphoric acid-based penetrant coating [5]. This system improves upon the oxidation protection systems of C/C by having a phosphorus based glass system beneath the cracks in the SiC layer. Cracks that are formed due to the thermal mismatch and cyclic conditions can be closed by the self-healing property of this specific phosphorus glass layer [20]. Multilayer antioxidant systems [21-32] are commonly utilized at moderate to elevated temperatures, but for C/C composites brake applications these techniques are not as economical as compared to the simple phosphate based systems designed for the application of lower to moderate temperature protection ranges.
Low to moderate temperature oxidation protection systems have a temperature protection range from 400° C. to approximately 900° C. [6, 33]. This margin is within the temperature range of typical aircraft brake applications. These protection systems usually contain glass formers of boron oxide and metal phosphate materials that are able to actively protect the composite in the low to moderate temperature range. Common weaknesses that are associated with phosphorus based anti-oxidants include sensitivity to moisture, elevated oxygen permeability, and high vapor pressure. These factors tend to cause Antioxidant Migration (AOM) onto friction surfaces, therefore leading to low frictional properties [18].
A patent application by Golecki [34] overviewed different fluidized glass materials such as phosphate glass, borate glass, silicate glass, and plumbate that can potentially protect carbon fiber or C/C composite materials from oxidation. These phosphorus based glasses include phosphates of aluminum, manganese, zinc, nickel, vanadium, and/or alkaline earth metals such as potassium, sodium, magnesium, calcium, and even lithium. One specific composition disclosed by Golecki is capable of impregnating and protecting a C/C composite material. The composition is 29 wt % phosphoric acid (H3PO4), 2 wt % manganese phosphate, 3 wt % potassium hydroxide, 1 wt % boron nitride, 10 wt % boron, and 55 wt % water.
Patents from Stover [35, 36] contain phosphorus based antioxidants that are capable of impregnating C/C composites and inhibiting the catalytic effect from anti-icing and de-icing agent contamination on the runways. The mixture was comprised of (a) phosphoric acid, (b) a metal phosphate, and (c) a C/C composite compatible wetting agent. The percent weight composition of each chemical was as follows; phosphoric acid 50-75 wt %, the metal phosphate is 25-50 wt %, and the wetting agent was around 0.3-3 wt %. Another weight percent composition was water 40-70 wt %, phosphoric acid 50-75 wt %, metal phosphate 25-45 wt %, and the wetting agent 0.3-3 wt %. Molar ratio of aluminum to phosphorus elements in the antioxidant mixture varied from approximately 0.2 to 0.8. The aqueous mixture was then applied by painting, dipping, or spraying the susceptible regions that are exposed to oxygen such as the inner and outer diameters of the rotor and stator discs.
A patent from Walker [5] contains a phosphoric acid based penetrant salt solution that is known as P13. The percent weight composition of each chemical was as follows: water 10-80 wt %, phosphoric acid 20-70 wt %, manganese phosphate 0-25 wt %, boron oxide 0-2 wt %, and an alkali metal mono-, di-, or tri-basic phosphate 0.1-25 wt %. The penetrant salt solution can also be applied to the composites surface by painting, dipping, or even spraying. The coated composite is then heat treated, at a temperature ranging from 500° C. to 900° C., so that solid char is produced from the coated solution. The barrier coating thickness varies between 1 and 10 millimeters thick according to the number of char cycles. Shelf life has been an issue with phosphoric acid based systems. However, the patent stated the shelf life of the product and the migration of phosphorus to the rubbing surface both increased substantially. It was also stated that this particular antioxidant system prevents catalytic oxidation by blocking the active sites on the carbon surface with metal phosphate deposits [37].
Wu [38-40] compiled a series of papers that studied the catalytic effect of potassium and calcium acetates on C/C composite aircraft brake materials. Exposure of the brake disc to catalytic materials, such as runway deicers, often leads to rapid wear and decomposition. Wu studied oxygen containing phosphorus groups that suppress the catalytic oxidation effect of C/C composites by blocking the active sites on the carbon surface. The salt-derived catalyst materials that were examined in the paper were potassium acetate and calcium acetate runway deicers. The catalytic effect of calcium acetate was almost completely suppressed by the deposited phosphorus groups.
The effects from potassium acetates were partially suppressed due to the superior wettability and mobility of potassium. Wu also studied the catalytic resistance effects of deposited boron oxide on C/C composites. It was documented that boron oxide was the deposited boron material located on the outermost surface of the carbon substrate. The boron doping study showed nearly a complete suppression of calcium acetate due to its poor ability to maintain direct contact with the carbon substrate. Boron oxide still showed little suppression of potassium acetate because of its great ability to maintain direct contact with the substrate s surface. Potassium's ability to migrate into any exposed surface region in order to maintain interfacial contact with carbon makes potassium acetates very caustic to C/C composite materials. Mazany [41] presented a phosphorus based antioxidant that infiltrates most open pores within the composite. The chemicals that are present in the antioxidant are as follows: phosphoric acid or an acid phosphate salt, at least one aluminum salt, and at least one additional metal salt. It was pointed out by McKee [42] that phosphates can deactivate many catalytic impurities from the carbon surface by converting them into inactive and stable phosphates. This patent also refers to an antioxidant that is more resistant to AOM by adjusting the metal to phosphate ratio. This stabilization of the phosphate material makes the antioxidant better suited, economically, for commercial C/C composite aircraft brakes.